1. Field of the Invention
This invention relates to a fibrous sheet material having improved surface smoothness and improved stretch characteristics and a method and apparatus for manufacturing it.
2. Description of the Related Art
Fibrous sheet base materials are conventionally used in tapes such as masking tape. In such tapes, and adhesive is applied to one side of the sheet of fibrous base material while a nonstick coating is applied to the opposite side. The coated sheet is then wound and cut in rolls.
In many applications, it is important that the tape be stretchable. For example, when masking tape is used to mask portions of an automobile being painted, the tape must be capable of stretching so that the tape may be smoothly applied along curved or irregular edges.
It is also important that such tapes have relatively smooth surfaces so as to minimize the amounts of adhesive and nonstick coating required to cover the tape, and to provide improved tape performance. For example, when tape is used for masking during painting operations, it is desirable that the side of the tape adhering to the object being painted be as smooth as possible in order to prevent paint seepage beneath the edge of the tape. Making the opposite nonstick surface smooth faciliates prolonged use of the tape because a smooth outer surface will not irritate the fingers and thumbs of workmen applying the tape for prolonged periods, thus helping prevent the "sore thumb" problem experienced by many car painters.
Stretchability in tapes has been conventionally achieved by creping the tape base material while it is still in a wet state as it is being produced on a paper machine. Conventional creping of masking tapes is described in U.S. Pat. No. 2,941,661 issued to Picard et al. In such creped tapes, creping is performed on a wet press roll with a thin and sharp creping blade so as to impart about 40 to 60 crepe lines per linear inch and to provide rounded crepe ridges on one side and a flat appearing reverse side. This creped structure is retained during subsequent drying of the tape base. However, this creping process is difficult to control and presents many problems in maintaining product uniformity and productivity. Additionally, a creped sheet by its very nature is not smooth and the best that can be hoped for is to make the crepes as fine, uniform and even as possible.
Improvement in stretch of paper webs has been achieved with the apparatus and process for treating web material disclosed in U.S. Pat. No. 4,551,199 issued to Weldon. According to Weldon, bulking and creping of a paper web is achieved by transporting a web into a differential velocity nip defined by a web support surface and an open mesh fabric pick-up member having voids therein. The pick-up member has a relative velocity slower than that of the support surface at the nip location. When the web is applied to the fabric pick-up member, the web is impressed into the voids of the fabric so as to emboss the web. Also, as the web approaches the nip, a deceleration of the web occurs due to the slower moving fabric filaments of the pick-up member causing the web to collapse on itself one or more times to form crepe folds. The succeeding folds in the web press against earlier folds, pushing them into the voids of the fabric. The size and number of folds are determined by the flexibility of the web and the magnitude of the relative velocity differential between the pick-up fabric and the transport member support surface.
Web material produced as described in Weldon has surface undulations and a caliper greater than is desirable for tape base materials. For fiber web sheets of a given thickness, as the amplitude of surface undulations decreases, the density of the sheet increases. Accordingly, apparent bulk, which is the inverse of sheet density, decreases as the amplitude of the undulation decreases. For a tape base material, it is advantageous for the caliper of the tape base material to be relatively thin, that is less than 0.008". Sheets of this thickness produced according the process disclosed in Weldon have an apparent bulk greater than 0.4 cal pts/lb ream which is indicative of folding and bulking much greater than what is desirable for tape base materials.